Electric starter device for an internal combustion engine

ABSTRACT

The invention relates to an electric starter for an internal combustion engine, having a thermal monitoring protector ( 4 ) for turning the starter ( 1 ) off when its limit operating temperature is reached. The starter is characterized by a device ( 5 ) for ascertaining a virtual operating temperature (T V ); the device ( 5 ) ascertains the virtual operating temperature (T V ) as a function of at least one operating parameter that affects the operating temperature of the starter ( 1 ).

The invention relates to an electric starter for an internal combustionengine.

PRIOR ART

In luxury cars, so-called tip starters are preferably used, whichautomatically perform the starting operation for the internal combustionengine by means of a starting pulse. The result is a decoupling betweenthe selection of starting and the starting event itself. Intip-starters, the starting event is thus controlled electronically andis ended at the earliest possible moment, for instance once the engineis at a predeterminable minimum rpm. Since the starting event is notsuccessful in all cases, provision must be made to assure that thestarting event will not be continued indefinitely, both to protect thestarter battery and to avoid a thermal overload on the electric starter.It has been found that in electric starters, the hottest points, withthe greatest potential risk, are the carbon brushes and the commutator.

To make a thermal monitoring protection for such starters feasible, itis known in the prior art to dispose a bimetallic switch in the regionof or next to the so-called terminal 45 (positive starter terminal).Disposing the bimetallic switch in the region of the critical (hottest)points, however, is either very difficult or even impossible inengineering terms. The bimetallic switches must accordingly be designedsuch that they react, i.e. switch, at a temperature at which it isassumed that a maximum possible limit operating temperature is occurringat the highest-risk points of the starter.

Such bimetallic switches are indeed simple in design and in theirprinciple of operation, but they are relatively large. Moreover, theirswitching performance involves to severe hysteresis. Besides, theactivation and deactivation temperature is highly subject to variation.Since the bimetallic switch cannot be made with the requisite lowimpedance necessary for the main starter current, the choice is made tohave the bimetallic switch trigger a relay, which switches the mainstarter current. This arrangement means an additional plug connection inthe relay circuit and a double line between the relay and the bimetallicswitch. Thus it contributes to increasing the resistance in the relaycircuit.

ADVANTAGES OF THE INVENTION

The electric starter for an internal combustion engine offers theadvantage over the prior art that because a virtual operatingtemperature of the starter is ascertained, sensor costs and installationspace, lines, and an increase in resistance in relay circuit no longeroccur. Moreover, turning the electric starter on and off does notinvolve hysteresis, and the activation and deactivation temperature canbe specific precisely. Finally, the electric starter of the inventionoffers the advantage that the problems of the difference in temperatureresponse between the sensor point and the actual monitoring point(brushes, commutator) no longer arise. These advantages are attained inthat a virtual operating temperature of the starter is ascertained, andit is provided that this virtual operating temperature is ascertained asa function of at least one operating parameter that affects theoperating temperature of the starter. It has been found that when avirtual operating temperature is ascertained as a function of anoperating parameter of the starter, a high degree of agreement withactual temperatures occurring at the various components is obtained. Inthis respect, the electric starter of the invention thus offers theadvantage that—without direct measurement—the temperature range ofinterest can be detected very precisely, and the temperature of theelectric starter can be ascertained precisely.

In a further feature of the invention, it is provided that in operationof the starter, that is, as the operating temperature of the starterrises, the virtual operating temperature is ascertained at least as afunction of the starter current Primarily it is the operating current ofthe starter that causes the heating of the starter. That is, if thisoperating parameter is taken into account in ascertaining the virtualoperating temperature, then a heating model can be made that veryprecisely reflects again the operating temperature of the starter as afunction of the ON time of the starter.

Preferably, when the operating temperature is falling, the OFF time ofthe starter is taken into account. That is, the virtual operatingtemperature is affected as a function of the OFF time. Hence if thestarter is not in operation, then from the OFF time—on the basis of thepreviously ascertained, higher virtual operating temperature—the coolingtemperature or temperature gradient is ascertained.

In a preferred embodiment of the electric starter, it is provided thatthe virtual operating temperature is ascertained as a function of areference temperature, in particular the ambient temperature of thestarter. Thus it can be provided that when the starter is in operation,the rising virtual operating temperature is ascertained on the basis ofthe ambient temperature. Conversely, in the cooling-down phase, it canbe stated that the virtual operating temperature cannot drop below theambient temperature.

It is preferably provided that the device has a means for detecting theambient temperature. It is thus possible to perform a calibrationbetween the virtual operating temperature and the ambient temperature,since the detection means ascertains the actual ambient temperature. Areference can thus be made between the virtual operating temperature andan actually measured temperature, especially the ambient temperature orthe reference temperature.

One exemplary embodiment is distinguished in that the virtual operatingtemperature is ascertained as a function of the current ratio(i/i₀)^(b). That is, a standardized starter current is taken intoaccount, where i is the actual starter current, i₀ takes a referencecurrent into account. The exponent b can be assumed to be astarter-specific parameter. Because the virtual operating temperature isascertained as the function of the current ratio, a reference can alsobe established for whether the starter is heating up very quickly or notso much. If because of temperature factors, for instance, the engine issubjected to an increased torque in the starting event, usually theconsequence is an increased starter current. The starter wouldaccordingly heat up much faster. This is taken into account inascertaining the virtual operating temperature as a function of thecurrent ratio.

To make it possible to graph the cooling down of the starter veryprecisely, it is preferably provided that during the OFF time of thestarter, increased cooling of the starter is assumed, if the virtualoperating temperature is especially high. In other words, if a hightemperature difference exists between the virtual operating temperatureand the reference temperature, in particular the ambient temperature,then the starter can dissipate its heat to the environment relativelyquickly. In the upper temperature range, the cooling down is thusspeeded up. This is taken into account with the provision according tothe invention.

If the OFF times last relatively long, it can be provided that areduction in the cooling gradient of the virtual operating temperatureis assumed. That is, as the virtual operating temperature falls, itapproaches the ambient temperature more and more slowly. This influenceis accordingly taken into account in the ascertainment of the virtualoperating temperature.

In a preferred exemplary embodiment, it can be provided that as afunction of the OFF time of the starter and of its instantaneous virtualoperating temperature, the virtual operating temperature is set equal tothe ambient temperature. For instance, if the starter is out ofoperation for a relatively long period of time, and if the modelcalculation assumes that the starter has already cooled down completely,but the virtual operating temperature is above or below the ambienttemperature, then—to make it possible to start the system again from areference point—the virtual operating temperature is set equal to theambient temperature. It is thus assumed here that after a certain OFFtime, the starter has the same temperature as the ambient temperature orreference temperature. This assures that any error that may have beenincorporated into the determination of the virtual operating temperatureis corrected at defined time intervals.

It can also be provided that as a function of the cooling gradient, thevirtual operating temperature is set equal to the ambient temperature orreference temperature. If the virtual operating temperature decreasesonly very slightly over time, then the virtual operating temperature canbe reset to the ambient temperature, since it can be assumed that theactual operating temperature of the starter is virtually identical tothe ambient temperature.

Alternatively, it can also be provided that as a function of the OFFtime and/or of the cooling gradient, the virtual operating temperatureis adapted in stages to the reference temperature or ambienttemperature.

Further advantageous features will become apparent from the dependentclaims.

DRAWING

The invention is described in further detail below in terms of anexemplary embodiment in conjunction with the drawing. The sole FIGURE,FIG. 1 is a block circuit diagram of an electric starter with a devicefor ascertaining a virtual operating temperature.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

In the block circuit diagram, the electric starter for an internalcombustion engine is identified by reference numeral 1. The starter ispreferably a so-called tip starter, which is triggerable via a controlunit that in response to a control pulse 3 actuates the starter 1 untilsuch time as the engine (not shown) that is to be started has reached apredetermined rpm. After that, the control unit turns the starter 1 offagain, to protect it from damage. The control unit 2 preferably has aprotective function to guard against misuse of the starter 1, so thatthe starter 1 cannot be activated while the engine is running, forinstance.

The block circuit diagram also shows a thermal monitoring device 4 forthe thermal monitoring protection of the starter 1. The monitoringdevice 4 preferably forms a unit with the control unit 2 and inparticular is integrated with the control unit 2.

The monitoring device is embodied as a device 5 for ascertaining avirtual operating temperature of the starter 1. As a function of atleast one operating parameter affecting the operating temperature of thestarter 1, the device 5 ascertains the virtual operating temperature.This creates a temperature model, which either comes quite close to theactual operating temperature of the starter 1 or is precisely equal toit. This operating parameter is for instance the main starter current i,which is detected by the device 5. The device 5 also detects whether thestarter 1 is on or off, or in other words has been activated by thecontrol unit 2 or put out of operation by it.

Once the starter 1 has been activated, in a heating model 6 the risingoperating temperature of the starter 1 is converted into the virtualoperating temperature of the starter 1, as a function of at least thestarter current i. The heating model 6 preferably functions inaccordance with a method that includes a heating gradient deltaT_(E)proportional to a current ration (i/i₀)^(b), in which i₀ represents areference current, especially the rater current, of the starter 1, and brepresents a starter-specific parameter and can be selected to suit theparticular model of starter involved. The heating gradient deltaT_(E) issent to an integrator and calibrator 7, which by integration from theheating gradient deltaT_(E) ascertains the virtual model temperature oroperating temperature T_(V) and sends it to the control unit 2. Based onthe virtual operating temperature T_(V) ascertained, the control unit 2decides whether the starter 1 can continue in operation or must be putout of operation, the latter being the case if the virtual operatingtemperature T_(V) reaches the maximum possible limit operatingtemperature of the starter 1.

If the starter 1 is not in operation, then in a cooling model 8 thethermal cooling gradient deltaT_(A) is ascertained and forwarded to theintegrator and calibrator 7. The cooling gradient deltaT_(A) is affectedby the magnitude of the absolute temperature T, which—at the onset ofthe cooling-down phase—is assumed to be the previously ascertainedvirtual operating temperature that was attained while the starter 1 wasin operation. The OFF time deltat of the starter 1 also affects thecooling gradient deltaT_(A). If the absolute temperature is high, thenthe cooling gradient deltaT_(A) increases. With an increasing OFF timedeltat, the cooling gradient deltaT_(A) decreases. Based on the coolinggradient deltaT_(A), the integrator and calibrator ascertains thefalling virtual operating temperature T_(V) and sends this on to thecontrol unit 2.

In order for instance at the onset of operation of the starter 1 toobtain a virtual starting operating temperature, the device 5 has atemperature detection means 9, for instance embodied as a temperaturesensor, that ascertains the ambient temperature of the starter 1. In acalibration model 10, the ambient temperature detected is compared withthe virtual operating temperature. The OFF time deltat of the starter 1is taken into account as well. If the starter 1 has been out ofoperation for a very long time, for instance, then in the calibrationmodel 10 it is determined that the virtual operating temperature and theambient temperature have the same value. The virtual operatingtemperature T_(V) thus ascertained, which is now equivalent to theambient temperature, is forwarded to the control unit 2. This assuresthat any model error that might have been incorporated into theintegrator and calibrator 7 will be corrected at certain time intervals.Alternatively, instead of the temperature detection means 9, thetemperature measured at the control unit 2 or some other referencetemperature is used, which is then utilized—like the ambienttemperature—for the calibration of the virtual operating temperatureT_(V).

The heating model 6 and the cooling model 8 preferably operate by aso-called description function, in which the individual influences thataffect the gradients deltaT_(E) and deltaT_(A) are each represented by afactor of suitable dependency. Since the main starter current i has avery great influence on the heating gradient deltaT_(E), the latter canbe occupied by a high factor, for instance. In particular, all theessential dependencies can be detected. These include in particular thedependency of the heating on the starter current, the dependencies onthe differential temperature between the commutator of the starter andthe ambient temperature, the dependency on the absolute temperature, andthe dependency on the time. In the temperature range of interest, thatis, essentially around the deactivation temperature range of thestarter, a high match can thus be achieved between the virtual operatingtemperature T_(V) and the actual measured values that—in anexperiment—are ascertained. It is accordingly found that the virtualoperating temperature T_(V) quite precisely reflects the actualtemperature of the starter 1, and in particular of the commutator and ofthe brushes.

Because the virtual operating temperature T_(V) is ascertained, or inother words is a quasi-assumed operating temperature of the starter,additional conditions can easily be taken into account in the heatingand cooling models 6 and 8. For instance, it can be advantageous for theturn-off temperature to be raised in an individual case, for instance toenable actuating the starter in emergency situations as well. Because ofthe calibration in the calibration model 10 after relatively long phasesin which the starter 1 is out of operation, any temperature model errorthat may have been incorporated is corrected at certain time intervalswith a guaranteed ambient temperature, so that the virtual operatingtemperature T_(V) is reset to a guaranteed value. All the models 6, 8and 10 are preferably structured as a description function withindividual factors in accordance with different dependencies. Theseparameters can therefore be adapted easily to given vehicle-specificproperties. Thus different starters with one and the same device 5 canbe used in different vehicle types, since the models 6, 8 and 10 caneasily be modified.

The models 6, 8 and 10 are preferably realized by means of amicroprocessor, in which the models can be converted both via analyticalequations that require little storage space and via tables with littlecomputation effort. In particular, it can be provided that the analyticequations and tables be ascertained in prior heating tests of thestarter 1, for instance to enable ascertaining the actual heating of thestarter 1 for a certain main starter current during a predeterminedlength of time.

What is claimed is:
 1. An electric starter motor for an internalcombustion engine, having a thermal monitoring protector (4) for turningthe starter (1) of when its limit operating temperature is reached,characterized by a device (5) for ascertaining a virtual operatingtemperature (T_(V)), wherein the device (5) ascertains the virtualoperating temperature (T_(V)) as a function of at least one operatingparameter that affects the operating temperature of the starter (1),wherein as the operating temperature of the starter (1) rises, thevirtual operating temperature (T_(V)) is ascertained as a function ofthe starter current.
 2. The starter of claim 1, characterized in thatwhen the operating temperature is falling, the OFF time (deltat) of thestarter (1) affects the virtual operating temperature (T_(V)).
 3. Thestarter of claim 1, characterized in that the virtual operatingtemperature (T_(V)) is ascertained as a function of a referencetemperature, in particular the ambient temperature of the starter (1).4. The starter of claim 1, characterized in that the device (5) has ameans (9) for detecting the ambient temperature.
 5. The starter of claim1, characterized in that the virtual operating temperature (T_(V)) isascertained as a function of the current ratio (i/i₀)^(b), in which i isthe starter current, i₀ is a reference current, and b is astarter-specific parameter.
 6. The starter of claim 1, characterized inthat at a high virtual operating temperature (T_(V)) during the OFF time(deltat) of the starter (1), increased cooling of the starter (1) isassumed.
 7. The starter of claim 1, characterized in that if the OFFtime (deltat) lasts relatively long, a reduction in the cooling gradient(deltaT_(A)) of the virtual operating temperature (T_(V)) is assumed. 8.The starter of claim 1, characterized in that as a function of the OFFtime (deltat) and of the instantaneous virtual operating temperature(T_(V)), the virtual operating temperature is set equal to the ambienttemperature.
 9. The starter of claim 1, characterized in that as afunction of the cooling gradient (deltaT_(A)), the virtual operatingtemperature (T_(V)) is set equal to the ambient temperature.
 10. Thestarter of claim 1, characterized in that as a function of the OFF time(deltat) or of the cooling gradient (deltaT_(A)), a graduated adaptationbetween the virtual operating temperature (T_(V)) and the referencetemperature is performed.